The long term objective of this project is the development and marketing of easily swallowed electrodes for temporary transesophageal cardiac pacing. This "Pill Electrode" can replace cardiac catheterization in a variety of diagnostic and therapeutic applications, including initiation and/or termination of tachycardias, short term maintenance of adequate rate in sinus bradycardia, acceleration of rate to produce stress during echocardiographic or radionuclide studies of ventricular function. With transesophageal instead of catheter pacing, these become out-patient procedures with a greatly reduced complexity and cost. The specific aims for Phase I are 1) determine an optimal design for the bipolar transesophageal pacing electrode, 2) implement the design in an electrode that can be enclosed in an ordinary pharmaceutical capsule like the present Arzco Pill Electrode for esophageal electocardiography, 3) develop new electronic circuits for the special non-polarizing stimulator to be used in this high-current, switched pacing/recording application, 4) fabricate a flexible, ultra-thin silicone rubber sheath to use with the Pill Electrode to improve fixation and provide for delivery of air and local anaesthetic, and 5) design a continuation study (Phase II) which will evaluate the electrode/stimulator system under clinical conditions, conceive new clinical applications, and develop a manufacturing and marketing plan for wide utilization of the technique.